Navigate to Data⇒Production⇒Subcontract

Double click on the subcontract operation you wish to select/change a Preferred Supplier for (or select the Operation and click Edit) and then select the Supplier tab

Select the part number a preferred supplier is to be set for (if the part required is not listed, add the part) and then select the supplier that is to be the preferred supplier If the supplier required is not listed, create the supplier) and then click Set Preferred Supplier

The chosen supplier is now listed in the Preferred Supplier field – click Update then Save